The present invention relates to thermostats for vehicles and, more particularly, to heavy duty hard seat thermostats having a soft mounting structure for permitting improved mounting of the thermostats in vehicle engines and improved thermostat performance.
Heavy duty vehicle thermostats are mounted in an opening in the engine block of a truck or other vehicle to control the flow of coolant under pressure through the cooling system of the engine. When the engine is cold, a valve element, usually a cylindrical sleeve, is seated to direct the pumping of coolant from an outlet of the engine back to the inlet, bypassing the radiator so that the engine can warm up. When the temperature of the coolant reaches a predetermined level, a thermally expansible element, such as a wax element, expands, lifting the cylindrical sleeve from its seat against the force of a spring to direct through the radiator an increasing amount of the coolant which circulates through the engine. The thermostats are typically mounted by the engagement of a flange extending radially from the seat of the cylindrical sleeve with a shoulder defined by a counterbore in a thermostat housing. The thermostat housing, which is usually a casting, is secured to the engine block by bolts to clamp the flange in place between the shoulder and the engine block. A gasket is usually positioned between the engine block and the thermostat housing for sealing purposes, the gasket extending into the space between the shoulder of the thermostat housing and the engine block.
The cylindrical sleeve of the thermostat and its seat are customarily made of metal, and the seat is rigid and has a hard surface. Although the mating surfaces of the sleeve and seat are lapped to provide a tight seal against the flow of coolant, sealing is less than perfect, and a small amount of coolant leaks past the engagement of the sleeve with its seat. If too much coolant leaks through, and flows through the radiator, it retards or prevents the warm-up of the engine, especially in cold weather. In many applications, leakage of 100 cubic centimeters of coolant per minute is considered negligible and does not affect the cooling of the engine. To provide an understanding how much leakage this is, it is noted that a generally radial scratch across a thermostat seat can result in leakage of 100 cc's per minute. In some applications, leakage requirements are more stringent, and any leakage above 50 cc's per minute is unacceptable. In a few cases, engine manufacturers have imposed a maximum limit of 10 cc's per minute leakage. An unacceptable number of units of previously known heavy duty vehicle thermostats have failed to meet the stricter leakage requirements.
In order to reduce the leakage or stop it completely, it has been known to cover the seat of the thermostat with an elastomeric material, a so-called "soft seat", so that better sealing is provided between the cylindrical sleeve and the seat. Although this arrangement works well initially in greatly reducing the leakage or stopping it completely, the elastomeric material on the seat is exposed to the flow of the coolant, hot liquid under high pressure, which is especially forceful when the valve just begins to open. As a result, the elastomeric material does not hold up over time and, in some cases, leads to the premature failure of the thermostat.